For measuring, or registering, a process variable of media flowing in lines, especially pipelines, especially for registering flow-dynamic and/or Theological, measured variables, often such inline measuring devices are applied, which utilize a vibration-type, i.e. vibratory, measurement pickup inserted into the course of the medium-conveying pipe, tube or hose line, and a measuring and operating circuit connected thereto, to cause, in the medium flowing through the pickup during operation, reaction forces, such as e.g. Coriolis forces corresponding to the mass flow rate, inertial forces corresponding to the density or frictional forces corresponding to the viscosity and to produce, derived from these forces, a measurement signal representing the particular mass flow rate, viscosity and/or density of the medium. The measurement pickup is, in such case, connected e.g. by means of flanges, medium-tightly, especially pressure-tightly, and mostly also permanently, with the line conveying the medium.
For operating the measurement pickup, especially also for the further processing or evaluating of the at least one measurement signal, such is, furthermore, connected with an appropriate measuring device electronics. In the case of inline measuring devices of the described kind, the measuring device electronics is, in turn, usually connected, via an attached data-transmission system, with other measuring devices and/or with an appropriate central computer, to which it sends the measured-value signals, e.g. via a digital data bus. Serving as data-transmission systems in such case are often, especially serial, bus systems, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, as well as the corresponding transmission protocols. The central computer can process the transmitted measured-value signals and visualize them as corresponding measurement results e.g. on monitors and/or convert them into control signals for corresponding actuators, such as e.g. magnetic valves, electromotors of pumps, etc. To accommodate the measuring device electronics, such inline measuring devices further include an electronics housing, which, as e.g. proposed in WO-A 00/36379, can be arranged remotely from the measurement pickup and connected therewith only over a flexible cable or which, as e.g. shown in EP-A 1 296 128 or WO-A 02/099363, is arranged directly on the measurement pickup, especially on a measurement pickup housing which houses the measurement pickup.
Especially, such inline measuring devices having a vibration-type measurement pickup are suited for the direct measuring of a mass flow rate balance, especially a mass flow rate difference, of two simultaneously flowing media conveyed in different medium-lines. Such balance measurements serve mostly to monitor the content of and/or the lack of leaks in, a container with connected system of lines by simultaneous measurement of the incoming flow of medium and the outgoing flow of medium. Corresponding cases of application for such balance measurements are to be found, for example, in the area of medical technology, especially in the areas of blood transfusion or dialysis, or in the area of paint technology, especially in the mixing of colors. Suitable balance measuring systems, which measure balances of two mass flows using inline measuring devices of the described kind, especially Coriolis mass flow measuring devices, are described e.g. in EP-A 441 328, EP-A 244 692, U.S. Pat. No. 6,457,372, U.S. Pat. No. 6,138,517 or U.S. Pat. No. 4,252,028.
For conveying the medium, the vibration-type measurement pickups disclosed therein all include two measuring tubes held in a frame of, for example, tubular or box shape. Each of these tubes is caused to vibrate—driven by an electromechanical exciter mechanism—for producing, during operation, the above-mentioned reaction forces. One of the measuring tubes is, in each case, intended for insertion into the course of a first medium-line conveying the, in the above sense, incoming medium, while the other measuring tube is provided for the, in the above sense, outgoing medium conveyed in a second medium-line. For registering, especially inlet-end and outlet-end, vibrations of the measuring tubes, the measurement pickups each have, furthermore, a physical-electrical sensor arrangement reacting to movements of the oscillating measuring tubes.
In the case of Coriolis mass flow measuring devices, the measurement of the mass flow rate of a flowing medium rests, in known manner, on allowing the medium to flow through each of the measuring tubes, which oscillate during operation laterally to an oscillation axis, whereby Coriolis forces are induced in such medium. These, in turn, cause inlet and outlet regions of the relevant tube to oscillate with phases which are shifted relative to one another, with the size of these phase shifts being a measure of the instantaneous mass flow rate in the measuring tube. The oscillations of each of the measuring tubes are, therefore, locally registered and converted into oscillation measurement signals by means of two oscillation sensors of the aforementioned sensor arrangement displaced from one another along the relevant measuring tube. The mass flow rate can then be derived from the relative phase shift.
In the case of balance measuring systems of the described kind, the measurement pickups have during operation, at least at times, two media simultaneously flowing through them and of character differing in at least one physical property, for example mass flow rate, density, viscosity and/or temperature. As a result of this, the measuring tubes can, during operation of the inline measuring device, significantly deviate from one another as regards their mechanical characteristics of oscillation, for example as regards the instantaneous oscillation amplitudes and/or the instantaneous oscillation frequencies, although they are constructed nominally practically identically. This can, as a result of the mechanical coupling between the individual measuring tubes, lead to considerable errors in the measured balance of the two medium streams, for example, thus, the measured mass flow rate difference, as discussed, for example, also in the already mentioned U.S. Pat. No. 6,457,372. For preventing or eliminating such measurement errors typical for balance measuring systems of the described kind, it is, furthermore, proposed in U.S. Pat. No. 6,457,372, to determine the oscillation amplitudes of the two, differently oscillating measuring tubes always separately, and, based on the individually measured oscillation amplitudes of each of the measuring tubes, to perform a suitable correction of the measured phase differences.
However, such a predominantly calculations-based compensation, which essentially only subsequently eliminates the measurement errors caused by the different oscillation amplitudes, leads, on the one hand, to an increased technical complexity as regards the construction of the exciter mechanism and sensor arrangement, as well as also regarding the construction of the measuring device electronics processing the measurement signals, as well as regards the hardware and also the software. Furthermore, it has been found, that the imbalances unavoidably associated with the non-uniform changes of the oscillation characteristics of the measuring tubes can lead to significant problems as regards the zero-point stability of the measurement pickup, which then are scarcely manageable any more by compensation measures limited essentially to measurement signal processing.